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# Don't understand the answer to a past paper question (aqa, june 2008) watch

1. I was looking at this question from a past paper and I don't understand how the mark scheme got to the answer.

Q: An aqueous buffer solution which contains 0.0550 mol of a different weak acid, HA, and 0.0250 mol of NaA in 100 cm3 of solution has a pH of 4.20.

The value of Ka for the acid HA is 2.87 × 10–5 mol dm–3 at 25 °C.

Deduce the pH of the solution formed when 100 cm∧3 of pure water are added to 100 cm∧3 of this buffer solution.

Surely the pH should go up because the concentration of H+ ions will have gone down. I may be wrong, if so could someone explain to me why?

Thanks
2. The pH of a buffer is independent of the volume, take a look at his equation:

{derived from the equilibrium constant formula}

the [A-]/[HA] means that the volume will always cancel so only the number of moles is relevant
3. So, in a normal solution volume does affect pH but not in a buffer. Why is that?

Also, I don't really understand that formula. I thought pH was -log [H+].
4. (Original post by fireice7)
So, in a normal solution volume does affect pH but not in a buffer. Why is that?
If you have just an acid, obviously diluting that acid makes the proton concentration fall. In a buffer the amount of salt (which you don't have with a simple acid) to acid becomes important.

Also, I don't really understand that formula. I thought pH was -log [H+].
You don't really need to understand it that pH formula only really works for strong acids where the dissociation is nearly complete. Buffers use weak acids so you have to start from here:

A bit of rearranging of this gets you the equation in my first post
5. (Original post by EierVonSatan)
The pH of a buffer is independent of the volume, take a look at his equation:

{derived from the equilibrium constant formula}

the [A-]/[HA] means that the volume will always cancel so only the number of moles is relevant
isn't it ?
6. (Original post by Lou Reed)
isn't it ?
Nah http://en.wikipedia.org/wiki/Henders...balch_equation

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